Ambient light trapping filter for cathode ray tubes



@HCH www? Filed Oct. l5. 1963 AMBIENT 4LIGHT TRAPPING FILTER FOR CATHODE RAY TUBES ful. 7 Cw nl L b nsw 2 NH 9 L 3 www. VV. LL E @www n@ N O neu T L M A H E V. 5 6 9 l 2 w 6 N, n

United States Patent O 3,215,777 AMMENT LIGHT TRAlllNG FILTER FOR CATHODF. RAY TUBES Vern E. Hamilton, Palos Verdes Estates, Calif., assignor to Douglas Aircraft Company, Inc., Santa Monica, Calif.

Filed Oct. 15. 1963, Ser. No. 316,355 14 Claims. (Cl. 178-7.85)

This invention relates to a light ray filter for use primarily with a diffuse radiant screen presenting reproduced images, and is directed particularly to such filter which will trap ambient light rays angularly directed toward such screen. The filter traps both diffuse and concentrated light, thus preventing all ofi-axis light from striking the screen and degrading the image contrast, and consequently improving the quality of the image which is viewed.

Cathode ray tubes are typical examples of diffuse radiant screens and are particularly susceptible to the effects of being struck by ambient light rays. As is generally known, a cathode ray tube has a glass front wall, the inner surface of which is coated with phosphors which are caused to glow momentarily when struck from the rear yby electrons from one or more electron guns in a selective manner to produce an image of some sort. When ambient light rays strike the frontsurfacc of the glass they are retracted inwardly and strike a multiplicity of phosphors. The light energy is reflected diffusely from these phosphors whether or not they are being activated by the electron gun. Theresult is that many ofthe shadow areas or low lights are illuminated and approach the brightness of -the high lights, thus degrading the contrast. The low lights may be made as bright as the high lights in some cases-and the picture may be completely lost.

Various schemes have'becn devised to prevent ambient light rays from striking the cathode tube glass or phosphors, either of which may be considered as the screen Large hoods have been placed around the tubes extending toward the viewers but these are very clumsy. Honeycomb cores have been placed in front of the tube but the honeycomb walls are not perfect absorbers so they reflect the light rays onward to strike the screen anyway. My copending application for patent on Ambient Light Filter, Serial No. 230,644,l filed October l5, 1962, discloses a novel construction which solves the problem.

Briefly, that novel construction comprises a filter body of transparent material in which are embedded a plurality of filter elements having a grid pattern. Each element constitutes a tier of alternating transparent and light absorbing material in closely spaced relation to constitute a multiplicity of light transimtting apertures bordered by light absorbing material. The grid pattern may be formed of generally parallel straight or wavy lines, or lines crossing each other to produce cells of varying shapes. The tiers are in generally parallel relation to each other and are spaced depthwise of the filter body with the light transmitting apertures in registry to constitute depthwise directed viewing cells. The axes of the cells may be normal to the plane of the filter body or at some other preselected angle, and may be parallel or divergent within limits.

Ambient light rays striking the surface of the filter body at acute angles beyond the desired viewing angles are retracted into the cells at an angle, striking one or anel-tl 3,215,717 Patented Nov.4 2, 1965 ice other of the depthwise spaced lines of opaque or light ab- I sorbing material and are absorbed thereby. The success of such filter results from the fact that the cells are very small in at least one lateral dimension and the lines forming the cell boundaries are extremely thin in the depthwise direction so that their edges present no appreciable grazing surface to reflect light rays onwardly. In fact, in a typical example the width of the apertures may be from .015 to 0.25 inch and the line -width from .004 to .007 inch. The line thickness may vary from .0001 to .0002 inch and the depthwise spacing between lines may be of the order of .010 inch. Such a filter with six filter elements is only one sixteenth inch thick.

The home television set presents a particular problem for a light trapping filter because direct or reflected light sources may be anywhere in the room and the user expects the filter to eliminate their-undesirable effects rather than require `him to move the set or block some of the light sources. This means that the filter must trap most of the light coming from above eye level or even from floor level, and also must trap light from larger angles at each side. This, in effect, calls for a closed figured grid pattern. In addition, the raster of the television tube presentsa series of alternating light and dark horizontal lines which are visible at all times when the tube is activated. The grid pattern must not combine with'these lines to produce a moire effect. In fact it is highly desirable to break up the existing horizontal line pattern of the raster to improve the image.

In another of my previously filed patent applications,

entitled Ambient Light Trapping Filter for Cathode Ray Tubes (Case I), Serial No. 306,473, filed September 4, 1963, I have described a filter which overcomes bot-h of these problems and greatly improves the image presented 'by the tube. In general, the filter is made up in the same way as those described in my first mentioned application. The grid pattern is formed by a first set of spaced parallel lines extending diagonally in one direction and a -second set of similar lines extending diagonally in another direction crossing the first set to present a repetitive pattern of diamonds, and the angles are so chosen that the width of each diamond is substantially greater than its height. In most cases the ratio is about two to one, or higher.

The limiting useful viewing angle is taken to -be the angle at which the viewer receives fifty percent transmission, and it is determined by the depth and shape of the viewing cells. With diamond shaped cells the useful viewing angle becomes a substantially elliptical cone, the ratio of height to width corresponding generally to the dimensional ratio of the diamond. The vertical eye level position normally varies only from about three to six feet above the floor, requiring only a relatively small vertical angle of view, but it is desirable to allow the viewer as much horizontal shift as possible, approaching the range of good viewing ofthe unfiltered picture. Thus the elliptical cone of vision is basically the most desirable. Once a satisfactory ratio is determined, the absolute angle can be varied. inversely with the depth of the cell.` Adding more tiers of filter elements narrows the angle and removing tiers widens it.

With the diamond grid, all portions of all lines are diagonal with respect to the raster lines. At larger angles no moireeffect is produced. The minimum angle below which the moire effect appears varies to some extent with the fineness of the lines, both on the grid and on the screen,

but it has been determined that the minimum angle is of the order of fifteen degrees.

Under some circumstances the acute angle between the grid lines themselves becomes the limiting factor. When this angle ybecomes too small the intersection appears to broaden out laterally and produce a noticeable dark spot which is repeated along all the vertical rows of intersections. In the eye of the vie-wer these spots appear tio blend and makea series of dark vertical dotted lines which may be disturbing as the horizontal raster lines. This may well occur while the angle between grid line and raster line is still several degrees larger than the critical angle.

I have determined that this difiiculty can be overcome by modifying the diamonds to form hexagons in which two sides are vertical and four sides are diagonal. With this construction there are no grid lines intersecting each other at acute angles. While a regular hexagon, oriented as stated, produces satisfactory results, all of the advantages of the diamond arrangement mentioned above are obtained by use of the preferred form in which the hexagon is depressed or attened. The vertical lines are preferably made shorter than the diagonal lines and the latter may be arranged to meet each other at obtuse angles substantially greater than 120 degrees. Their angles with respect to the raster lines can thus be reduced tothe minimum. The co'ne of vision can be made to correspond very closely with that produced by the diamond grid arrangement. It is possible with either arrangement to attain horizontal useful viewing angles about twice the magnitude of the vertical useful viewing angles.

As indicated above, the arrangement described eliminates the moire pattern problem without sacrificing. any of the improvement in contrast and clarity resulting from the use of the space lattice type filter. Another important advantage is that the hexagons give the appearance of acting in combination with the horizontal raster lines to produce a pleasing half-tone effect much the same as in newspaper and magazine printed pictures, and completely eliminate the unpleasing effect of the horizontal` lines in the raster.

Various other advantages and features of novelty will become apparent as the description proceeds in conjunction with the accompanying drawings, in which:

FIGURE l is a front perspective view of a cathode ray tube of a television set with the filter of this invention, a part being broken away, mounted adjacent the tube, the housing being shown in phantom lines;

FIGURE 2 is a greatly enlarged detail view in elevation of a portion of a diamond grid lter; and

FIGURE 3 is an enlarged fragmentary front elevational view of the screen with raster'lines thereon, partly covered by a portion of the filter with a hexagon grid formation.

A conventional type of television set is rather schematically illustrated in FIGURE 1 by a cathode ray tube 10 mounted in a housing 12 shown in phantom lines. The tube includes a front wall 14, on the inner face 16 of which is the usual phosphor layer 18. Electrons streaming from the electron gun, not shown, at the rear of the tube strike the phosphors selectively to produce image signals which pass through the glass wall of the tube and are emitted forwardly, o r to the right as viewed in FIGURE 1. Any light source, such as source 20, emits light rays in all directions and those such as ray 22 strike the front face of the tube and are refracted inwardly, if the angle is acute enough, to strike the phosphor screen. As explained in more detail in my previously filed case mentioned above, the retracted rays are reflected by the phosphors and degrade the image contrast. Rays striking at the more obtuse angles are reflected off the surface of the tube wall, and some of these latter travel toward the viewer and further degrade the image.

To overcome these disadvantages an ambient light trapping filter 24 is provided in the form of a generally planar panel of transparent material mounted in front of the tube face, substantially normal to the tube axis, and in proximity to the tube wall 14. The panel may be a single sheet of material but in the presently preferred form 1t 1s made up of a plurality of layers cemented together in a unitary manner. Each layer is provided on one or both faces with a filter element consisting of a tier of alternate transparent and highly light absorbent areas in a grid-like pattern forming a multiplicity of light passages. The grid patterns of the various layers are in depthwise registry to produce a multiplicity of depthwise directed viewing cells. Off-axis light rays such as 22, striking the front face of the filter and refracted into the viewing cells, strike the light absorbent material at nearly normal angles and are absorbed thereby. The material at the margins of the viewing cells is so thin as to present substantially no grazing surface, so the rays are not reflected onwardly toward the screen. v

Because of the scanning operation of the tube a series of horizontally extending light lines and dark lines 26 appear on the screen 18 at all times when the tube is activated. This is commonly referred to as a raster. When a space lattice type ambient light trapping filter is employed which includes horizontal lines in its grid pattern, these horizontal lines combine with the horizontal raster lines to produce a moire effect which is annoying and confusing and tends to distort the image which is being viewed. A filter for a home television set must overcome this difficulty and also put minimum restrictions on the location of light sources while allowing maximum range of viewing positions both vertically and horizontally. This is accomplished by the diamond grid filter described in detail in my previous patent application.

The diamond grid is highly satisfactory throughout a very wide range of width to height ratios, and is relatively easy to produce because it consists of continuous, straight, parallel lines. However, as maximum ratios are approached, the included angles at' the lateral points of the diamonds become more and more acute and the intersections appear to broaden out and form enlarged or more intense dark spots. This optical effect which appears in actual operation has been illustrated as well as possible in FIGURE 2, which shows a portion of a diamond grid filter 25 to a greatly enlarged scale.

Filter 25 bears, throughout its thickness, a plurality of -tiers of grid patterns `in diamond form made up ef a multiplicity of substantially straight and parallel, spaced lines 28 extending upward and to the right as seen in FIGURE 2 and a multiplicity of similar lines 30 extending upward and to the left. As the width to height ratio is increased, the angle A between the lines decreases. At some minimum angle, varying with the actual and relative widths of the lines and spaces, the intersections seem to enlarge or intensify and take on the appearance of black dots 32 which tend to merge into dotted lines extending up and down over the whole area of the filter. These lines appear to be heavier than the raster lines and therefore produce an effect which distracts the viewer by presenting a highly visible grid which degrades the image. This is so despite the fact that the filter grid lines themselves, 28 and 30, are so fine as to be unresolvable by the human eye at the normal viewing distance. These grid lines,` as previously pointed out, usually range in width from about .004 inch to about .007 inch, and very seldom reach a width of .O10 inch.

It has been determined that this undesirable effect can be eliminated by eliminating the acute angled intersections and replacing them by short vertical lines, resulting in hexagon shaped cells 40 having two vertical sides 34 and four diagonal sides 36, 38, as best illustrated in FIGURE 3. With these hexagonal cells -it is now possible to reduce the vertical angle between the diagonal grid lines and the horizontal raster lines to the minimum value which avoids the moire effect. This, in turn, makes possible the maximum ratio of width to height of the individual cells, and thus gives the viewer maximum freedom of lateral viewing position while still getting the benefit of closed-figure viewing cells. The benefit of such cells, as previously mentioned, is that they allow maximum freedom of positioning of light sources.

It will be seen that with the hexagon grid pattern as disclosed the angles between the diagonal sides or lines 36 and 38 are very obtuse, always being substantially more than 120 degrees. The angles between lines 36, 38 and lines 34 are always more than 90 degrees even with the maximum lateral stretch. When the length ratio of diagonal to vertical lines is about 2 to 1 the width to height ratio of the viewing cells 40 can readily be made at least 2 to 1. By further increasing the length ratio it is possible to attain a width to height ratio of about 4 to 1,

' which adequate for all ordinary circumstances. Using a line width range of about .004 inch to about .007 inch which has been found to be eminently satisfactory, the dimensions of an optimum hexagon cell are of the order of .020 inch for the length of the vertical sides and .060 inch for the length of the diagonal sides. Good results are obtained with length ratios of about 2 to 1 to about 4 to 1. Just as in the case of the diamond grids, once the width to height ratio of the cell has been determined for the particular usage, the absolute angles of the cone of vision will be a function of the effective depth of the filter.

The hexagons must be oriented as shown in the drawing and explained above. If the across-points direction is lateral then two sides of every cell will be horizontal. Even though these horizontal lines will be discontinuous they will still produce a moire pattern, and the more the cells are flattened the longer each horizontal line will be. In addition, as the cells are flattened the angles beproduce some detrimental effect on the image being viewed.

However, because of the closed figure grid lpattern the restrictions on light source location are minimized and a very satisfactory viewing range is achieved.

It will be apparent to those skilled in the art that various changes and modifications may be made in the construction and arrangement of parts and in the method of manufacture without departing from the spirit of the invention and it is intended that all such changes and modifications shall be embraced within the scope of the following claims.

I claim:

1. The combination of a cathode ray tube having a viewing screen from which image light emanates with a space lattice type ambient light trapping filter`mounted in front of and in proximity to said screen, the general planes of said screen and said filter being'substantially parallel; said screen, when activated, presenting a series of alternating light and dark horizontal lines constituting a horizontal raster; said filter comprising a thin, transparent panel bearing, throughout its thickness, a plurality of depthwise spaced tiers of filter elements, each in the form of a light trapping grid pattern of highly light absorbent material; said grid pattern consisting of narrow lines arranged to form a multiplicity of adjoining hexagonal frames, each enclosing a transparent area serving as a light passage; each frame being oriented so that four of its sides are diagonal lines and two of its sides are vertical lines, with the diagonal lines at sufficient angles to the horizontal lines of the raster pattern to prevent the formation of a moire effect between the grid lines and the raster lines; the light passages of successive grid patterns being in depthwise registry to provide a multiplicity of depthwise directed viewing cells; said depthwise spaced grid lines acting individually to trap and absorb off-axis ambient light rays entering said cells at various angles and striking said grid lines at nearly normal angles.

2. The combination as claimed in claim 1; the angles between the diagonal sides of the hexagons and the horizontal raster lines being of the order of 15 degrees.

3. The combination as claimed in claim 1; the width to height ratio of said hexagons being in the range from about 2 to 1 to about 4 to 1.

4. The combination as claimed in claim l; the length ratio of said diagonal lines to said vertical lines being in the range from about 2 to l to about 4 to 1.

5. The combination as claimed in claim 1; the diagonal lines having a length of about .060 inch and the vertical lines having a length of about .02() inch.

6. The combination as claimed in claim 1; the angles at the intersections of adjoining diagonal lines being at least degrees.

7. A space lattice type ambient light trapping filter adapted to be mounted in front of and in proximity to a cathode ray tube viewing screen presenting, when activated, a series of alternating light and dark horizontal lines constituting a horizontal raster, comprising: a thin, transparent panel adapted to be mounted in a generally vertical plane; said panel bearing, throughout its thickness, a plurality of depthwise spaced tiers of filter elements, each in the form of a light trapping grid pattern of highly light absorbing material; said grid pattern consisting of narrow lines arranged to form a multiplicity of adjoining hexagonal frames, each enclosing a transparent area serving as a light passage; each frame being oriented so that four of its sides are diagonal lines and two of its sides are vertical lines, all with respect to a horizontal reference line, with the diagonal lines at sufficient angles to the reference line to prevent the formation of a moire effect between the grid lines and the horizontalfraster lines of a viewing screen; the light passages of successive grid patterns being in depthwise registry to provide a multiplicy of depthwise directed viewing cells;

said depthwise spaced grid lines acting individually to trap and absorb off-axis ambient light rays entering said cells at various anglesv and striking said grid lines at nearly normal angles.

8. A- filter as claimed in claim 7; the angles between the diagonal sides of the hexagons and the horizontal reference line being of the order of 15 degrees.

9. A filter as claimed in claim 7; lthe width to height ratio of said hexagons being in the range from about 2 to 1 to about 4 to 1.

10. A filter as claimed in claim.7; the length ratio of said diagonal lines to said vertical lines being in the range from about 2 to 1 to about 4 to 1.

11. A filter as claimed in claim 7; the diagonal lines having a length of about .060 inch and the vertical lines having a length of about .020 inch.

12. A filter as claimed in claim 7; the angles at the intersections 4of adjoining diagonal lines being at least lines constituting a horizontal raster, comprising: a plurality of thin filter body layers of transparent material laminated and cemented face to face with transparent cement to form a substantially unitary filter body panel; a filter element arranged between adjoining faces of each adjacent pair of layers; each of said filter elements comprising a generally planar tier of alternating loci of transparent and highly light absorbing material arranged to provide a multiplicity of light transmitting apertures in close proximity to each other and separated by grid lines of light absorbing material to form a grid pattern of hexagonal frames, each enclosing a transparent area; each frame being oriented so that four of its sides are diagonal lines and two of its sides are vertical lines, all with respect to a horizontal reference line, with the diagonal lines at suicient angles to the reference line to prevent the formation of a moire effect between the grid lines and the horizontal raster lines of a viewing screen; the light transmitting apertures of successive grid patterns being in deplhwise registry to provide a multiplicity of dci'tthwise` 5 ing said grid lines at nearly normal angles.

References Cited by the Examiner UNITED STATES PATENTS 6/60 Beers -178-7.82 6/62 Nixon 88-.1

DAVID G. REDINBAUGH, Primary Examiner. 

1. THE COMBINATION OF A CATHODE RAY TUBE HAVING A VIEWING SCREEN FROM WHICH IMAGE LIGHT EMNATES WITH A SPACE LATTICE TYPE AMBIENT LIGHT TRAPPING FILTER MOUNTED IN FRONT OF AND IN PROXIMITY TO SAID SCREEN, THE GENERAL PLANES OF SID SCREEN AND SAID FILTER BEING SUBSTANTIALLY PARALLEL; SAID SCREEN, WHEN ACTIVATED, PRESENTING A SERIES OF ALTERNATING LIGHT AND DARK HORIZONTAL LINES CONSTITUTING A HORIZONTAL RASTER; SAID FILTER COMPRISING A THIN, TRANSPARENT PANEL BEARING, THROUGHOUT ITS THICKNESS, A PLURALITY OF DEPTHWISE SPACED TIERS OF FILTER ELEMENTS, EACH IN THE FORM OF A LIGHT TRAPPING GRID PATTERN OF HIGHLY LIGHT ABSORBENT MATERIAL; SAID GRID PATTERN CONSISTING OF NARROW LINES ARRANGED TO FORM A MULTIPLICITY OF ADJOINING HEXAGONAL FRAMES, EACH ENCLOSING A TRANSPARENT AREA SERVING 